The operation of electric motors can be characterized by their input frequencies (measured in Hertz (Hz)) and corresponding output speeds (measured in revolutions per minute (RPM)). The input frequency to a variable speed electric motor can be varied to change the output speed, which allows the same electric motor to be used with, e.g., different fan blades rated at different speeds and load points.
Fan blades may experience resonance at certain RPM. This resonance can be amplified into mechanical vibrations which may cause damage to the fan blades as well as unwanted noise. One solution is to avoid the RPM range associated with resonance by not allowing the electric motor to operate within that range. However, the electric motor may require an RPM within the disallowed RPM range in order to maintain a certain amount of power or torque, in which case disallowing the required RPM may result in even more noise or in the electric motor exhibiting “hunting” behavior, i.e., repeatedly jumping back and forth over the disallowed RPM range between a higher RPM that is not allowed and a lower RPM that is too slow. More specifically, a motor controller may command the electric motor to operate at a desired RPM that is just above the disallowed RPM range. However, load and/or temperature effects may prevent the electric motor from achieving the desired RPM, and may cause it to achieve an actual RPM that is within the disallowed RPM range, which then causes it to drop to an allowed actual RPM that is just below the disallowed RPM range. When the motor controller senses that the electric motor's actual RPM is substantially below the desired RPM, it commands the electric motor to return to the desired RPM which the electric motor cannot achieve. This cycle of jumping back and forth around the disallowed RPM range continues until the limiting operating conditions change so that the electric motor is able to achieve the desired RPM.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.